Microbially produced plastics, particularly a polyhydroxyalkanoate (hereinafter sometimes referred to as PHA) is a thermoplastic polyester which is produced and accumulated as an energy storage substance in cells of many microbial species. PHA produced by microbes using natural organic acids and fats as a carbon source is completely biodegraded by microbes in the soil and water and is taken into the natural carbon cycle process, so that PHA is an environmentally compatible plastic material with almost no adverse effect on the ecosystem. In recent years, synthetic plastics have become a serious social problem from the viewpoints of environmental pollution, waste disposal, and petroleum resources, and PHA has attracted attention as an environmentally friendly green plastic and its practical application has been in great demand. Also in the medical field, it is thought that PHA can be used as a biocompatible plastic such as an implant material which is not required for collection, a drug carrier and so on. Thus, PHA is expected to be put into practical use.
However, there is a problem that PHA produced by microbes is colored during the production thereof, such as during the culturing process. Patent Literature 1 (PTL 1) describes a method in which an aqueous suspension of a microbe cell containing PHA produced by a microbe is subjected to enzymatic treatment, then an alkali and a surfactant are added, and physical crushing treatment is carried out at a relatively low temperature, thereby to obtain a PHA having a yellowness index (YI value) of 15.0 or less. However, there is room for improvement in terms of processability, because treatment conditions of treatment temperatures, etc., as well as the type and amount of additives used in the treatment are limited in order to prevent the molecular weight of PHA from decreasing. Furthermore, this method was difficult to put into practical use because the method requires a large amount of agents and large-scale facilities for improving the color tone of PHA.